Frequency converter



June 28, 1938. w. A. HARRIS FREQUENCY CONVERTER 2 Sheets-Sheet 2 7 Filed March 9, 1937 T U W w E BAMNC/NG 00M vyvvvv III El AAAAAA I V'VVVV 5 056711.470? BALANCING COIL INVENTOR WILL/AM A. HARRIS WW ATTORNEY Patented June 28, 1938 UNITED STATES FREQUENCY CONVERTER William A. Harris, Nutley, N. J.,

assignor to Radio Corporation of America, a corporation of Delaware Application March 9, 1937, Serial No. 129,777

6 Claims.

This invention relates to vacuum tub-e circuits and more particularly to such circuits wherein oscillations from two sources or channels are united through a suitable mixing means to provide a beat or common output signal.

It is customary, in certain types of receivers, such as a super-heterodyne circuit, to employ a multi-grid converter or frequency changing tube to combine two different signals, such as a radio signal and the local oscillator signal, and to apply the resultant beat frequency signal to an output circuit, such as the intermediate frequency amplifier. In these frequency changing circuits difficulties have heretofore been experienced in preventing interaction between the circuits. Where the multi-grid tube is used as a beat frequency oscillator, it is of special importance that the two oscillator circuits be unaffected by each other, in order to prevent the circuits from pulling together when the separation of the frequencies is small.

Accordingly, one of the objects of the present invention is to provide an improved signal transmission system wherein oscillations from two signal channels may be combined and provided with an output circuit, without interaction the one upon the other, whereby said channels function independently of each other.

A more specific object is to provide an improved detector-oscillator system for a heterodyne type of receiver employing a singlev pentagrid electric discharge device as a combined detector and oscillator.

A further object is to provide an improved beat frequency oscillator wherein two signal channels, either or both of which may be a local regenerative oscillator, are mixed in a single multi-grid electric discharge device, the beat frequency being obtained in an output circuit, while each signal channel is maintained electrically independent with respect to the other.

In accordance with the invention, two signal channels are combined in a pentagrid electron discharge device which has, in the order named,

55 ing or reducing the effect of the signal voltage a cathode, a first signal'grid, an anode-grid, a.

from one signal channel upon the other signal channel, there is provided, in one embodiment, a coil in series with the output anode circuit for inducing in said other channel a voltage which neutralizes or balances out the components produced in the signal grid directly associated with said other channel by the first channel. According to another embodiment of the invention, there is provided in circuit with each signal channel, a separate coil associated with the electronic path of the electric discharge device for eliminating or reducing the effect of the signal voltage on said channel produced by the other channel. Stated in other words, and more specifically in accordance with the invention, two preferably negative and high input impedance signal grids are arranged in a common envelope of an electric discharge device to control, in series relation to each other, a common electronic stream, the grids being shielded one from the other by another electrode, or other electrodes, at a positive potential, said signal grids being utilized to operate at different signal frequencies, there being provided means external of the electric discharge device for preventing interaction of the input circuits upon one another.

One feature of the invention lies in the use of an additional balancing coil which is magnetically coupled to the output anode circuit for elimihating or reducing the undesired effects of the oscillator frequency voltage in the output.

A more detailed description of the invention follows, accompanied by drawings wherein:

Fig. 1 illustrates the invention in simplified form, as applied to a superheterodyne receiving system; 7

Fig. 2 illustrates another embodiment of the invention as applied to a beat frequency oscillator having two regenerative local oscillators connected to the signal grids and supplied with energy from the single electric discharge device; and

Fig-3 illustrates a modification of the embodiment of Fig. 1 provided with an additional coil coupled to the output circuit for reducing the undesired effects of the oscillator frequency upon the output circuit.

,Referring to Fig. 1, there is shown coupled to a combined first detector and oscillator pentagrid electric discharge device I, .a radio frequency signal input channel 2 connected to a signal grid G4, a local oscillatory regenerative circuit 3 coupled to a signal grid GI through a grid leak and condenser combination 4, and an output circuit comprising, a condenser 5 and an inductance coil 6 coupling the anode P on one end to an intermediate frequency signal output channel I, and on the other end to the oscillatory circuit 3 through a balancing coil 8.

The device I is a multi-grid device of the 2A1 or I5A'l type having within an evacuated envelope the usual cathode K heated by heater element H and an anode P between which, in series relation, along the electronic stream, are interposed five other electrodes or grid elements GI, G2, G3, G4, and G5, being respectively a negative signal grid, a positive anode-grid, a positive screen grid, a negative signal grid, and a positive screen grid, both screen grids G3 and G5 being directly connected together, as shown. The second signal grid G4 is connected with the signal or radio frequency input channel through the secondary Ill of the input transformer II, and the grid circuit therefor is returned to the cathode K through ground and a suitable biasing resistor I2.

The first control grid GI is coupled through a suitable grid leak 4 to the local oscillatory circuit 3 which is regeneratively coupled back through coil 22 to the positive anode-grid G2, for obtaining therefrom suitable exciting potentials. Local oscillatory circuit 3 is tuned to a desired frequency in order to produce in the output circuit 5, 6 the desired intermediate frequency.

The screen grid electrodes. G3, G5, are interposed between the control electrodes GI and G4, and also between G4 and the output anode P, and are operated at a positive potential from a suitable source, as indicated.

It should be noted that any increase in the positive potential on signal grid G4 will increase the flow of electrons to the output-anode P. We can thus say that the transconductance between signal grid G4 and output anode P is positive, because of increase in electron flow to anode P upon a positive increment of voltage on G4. Expressed in another way, SmG i, P is positive. Simultaneously an increase in the positive potential on G4 will cause a decrease in the flow of electrons to the anode-grid G2. It will thus be seen that the transconductance between signal grid G4 and anode-grid G2 is negative. Putting it another way, SmG l, G2 is negative. on the other hand, an increase in the positive potential on GI will cause an increase in the electron flow to the output-anode P and also an increase in the electron flow to grid-anode G2. Thus, SmGI P and SmGI, G2 are both positive. To overcome the reaction of the radio frequency input signal applied to signal grid G4 on the oscillator circuit 3 connected to grid GI, there is provided a balancingcoil 8 which is conductively connected to the output-anode P through coil 6 and is magnetically coupled to the inductance of the oscillatory circuit 3 for producing in the local oscillatory circuit, neutralizing or balancing potentials of such phase and magnitude as to eliminate or reduce the effect of the radio frequency signal voltage on the oscillatory circuit applied to the oscillatory circuit through the interaction in the device. Coil 8 tends to increase the desired component of voltage of oscillatory frequency while eliminating the undesired component of signal frequency because of the positive and negative transconductances mentioned above.

Fig. 2 shows a beat frequency oscillator comprising two local oscillatory circuits generating different frequencies and so coupled to the electric 75.;discharge. device 5 as to prevent one oscillatory circuit from interacting upon the other, thus obviating any tendency for the two oscillators to pull or entrain together. This tendency of the oscillatory circuits to pull together is especially noticeable when the frequencies generated by the separate channels are very close together. The principle of operation is similar to that which has been described above in connection with Fig. 1.

In Fig. 2 both oscillatory circuits A and B are shown as consisting of a parallel combination of inductance coil and condenser, the inductance coil of each circuit being inductively coupled to three other coils, one of which is connected to a signal grid, the second of which to the outputanode circuit for obtaining regenerative potentials therefrom, and a third of which is provided for producing the balancing or neutralizing effect to overcome interaction caused by the device.

Oscillatory circuit A is coupled through coil I5 to the signal grid G4 of the electric discharge device I, and obtains its regenerative potential from the output anode P through coil I 6, and also from anode-grid G2 through coil Il. When coils I6 and I! are connected in this regenerative sense, a voltage on signal grid GI produces an eifect in circuit A, through the anode P and coil I B. At the same time the voltage on signal grid GI will produce an opposite effect in circuit A, through the anode-grid G2 and coil Il. These two efiects are made to be equal in magnitude by adjustment of the coupling or number of turns on the coils and since the effects are of opposite phase, the net voltage produced oncircuit A by the voltage on signal grid GI is zero. Oscillatory circuit B is shown coupled to signal grid GI by means of coil 20, and obtains its regenerative potentials from the anode P through coil l8 and also from anode-grid G2 through coil I9. When coils I8 and I9 are connected in this regenerative sense, a voltage on grid G4 produces an effect in circuit B through the anode P and coil l8. At the same time the voltage on signal grid G4 will produce an opposite eifect in circuit B through the anode-grid G2 and coil I9. These two effects are made to be equal in magnitude and since they are of opposite phase, the net voltage produced in circuit B by the voltage on signal grid G4 is zero.

The foregoing neutralizing effects of one signal channel on the other was obtained by coupling coils I 6 and H in opposite senses to circuit A, and by coupling coils I8 and I9 in the same sense to circuit B.

In practice, when the system of Fig. 2 is set up, the various coils are coupled to the circuits A and B to give the desired amount of regeneration, whereupon adjustments are made first in one circuit, either A or B, to produce the balancing or neutralizing voltages to offset the effect of the other circuit, after which the adjustments are made in the other circuit to produce balancing or neutralizing voltages to offset the effect of the first circuit.

Although oscillatory circuits A and B are shown inductively coupled to the control grids G4 and GI, respectively, it is to be understood that, if desired, such inductive coupling coils I5 and 2!! may be eliminated and a direct coupling provided between the circuits A and B and the control grids in a manner somewhat similar to that shown in Figs. 1 and 3.

One advantage in the particular arrangement shown resides in the fact that the balancing circuits I1 and I9 tend to increase the desired component of voltage in the associated oscillatory circuits while eliminating the undesired component of signal frequency.

For deriving the beat frequency output from the oscillator there are shown a pair of leads but it will be apparent that the invention is not limited to this particular manner of obtaining the difference frequency since other means of coupling an output circuit to the oscillator may be used instead, such as by coupling to anode-grid G2.

Fig. 3 illustrates an embodiment very similar to Fig. 1 with the addition of a second balancing coil 2! arranged in series with coil 8 and inductively coupled to the output-anode coil 6 and the intermediate frequency output 1 for eliminating or reducing the effect of undesired components of the voltage from the oscillatory circuit 3 in the output circuit.

In operation, the circuit of Figure 3 is substantially like that of Figure 1 except for a more complete neutralization of the effects of one signal circuit upon the other occasioned by the connection of the second balancing coil 2| which is arranged in series relation with feedback coil 22 connected to the anode-grid G2 and in magnetic coupling relation to the. output circuit 6. Whatever reaction between the two input signal circuits that may appear in the output 6 is transferred through the balancing coil 2| to the feedback coil 22 and anode-grid G2 in such phase as to neutralize such reaction.

What is claimed is:

1. In combination, an electron discharge device having a cathode, first, second, third and fourth grids, and a plate, in the order named; a first signal channel applied between said first grid and cathode; a second signal channel applied between said fourth grid and cathode; means for applying suitable positive potentials to said second and third grids and to said plate, said first and fourth grids being at a negative potential with respect to said cathode; and a coil in series relation with the anode of said device and magnetically coupled to said first signal channel for balancing out the effect in said first channel of the alternating current voltage applied to said fourth grid by said second channel; and another coil in series with said first coil and anode and magnetically coupled to the second signal channel for balancing out the effect in said second channel of the alternating current voltage applied to said first grid by said first channel.

2. A system in accordance with claim 1, characterized in this that said first signal channel is an oscillatory circuit for locally generating oscillations, and said second signal channel is an external source of radio frequency signals.

3. A system in accordance with claim 1, characterized in this that said first and second signalchannels are oscillatory circuits for locally generating separate oscillations of different frequencies.

4. In combination, an electron discharge device having within an evacuated envelope a cathode, first, second, third, fourth and fifth grids, and a plate, in the order named; a first signal channel for applying external radio frequency signals to said fourth grid; a second signal channel comprising an oscillatory circuit for locally generating oscillations connected to said first grid, a regenerative feed-back circuit between said second grid and cathode and coupled to said oscillatory circuit; means for applying suitable positive potentials relative to said cathode to said second, third and fifth grids and to said plate, said first and fourth grids being at a negative potential relative to said cathode; an output circuit comprising an inductance coil and condenser combination coupled between said plate and cathode; and a coil in series with said output inductance coil and coupled to the oscillatory circuit for balancing out the efiect in said oscillatory circuit of the radio frequency signals applied to said fourth grid; and another coil in series with said feed-back path and coupled to said output circuit for balancing out undesired effects of said oscillator frequency voltages in said output circuit.

5. In combination, an electron discharge device having a cathode, first, second, third and fourth grids, and a plate, in the order named, there being a positive transconductance between each of said first and fourth grids and said plate, a positive transconductance between said first and second grids and a negative transconductance between said second and fourth grids, a first signal channel applied between said first grid and cathode; a second signal channel applied between said fourth grid and cathode; means for applying suitable positive potentials to said second and third grids and to said plate, said first and fourth grids being at a negative potential with respect to said cathode; and a coil in series relation with the anode of said device and magnetically coupled to said first signal channel for balancing out the effect in said first channel of the alternating current voltage applied to said fourth grid by said second channel; and another coil in series with said first coil and anode and magnetically coupled to the second signal channel for balancing out the effect in said second channel of the alternating current voltage applied to said first grid by said first channel.

6. In a signalling system, the combination with two input signal circuits and a common output circuit therefor, of an electric discharge device having two signal grids connected with said input circuits, an anode connected with said output circuit, and a screen electrode between said signal grids, one of said signal grids being more adjacent to the anode than the other, means for negatively biasing each of said signal grids and applying a positive potential to said screen electrode, a positive anode-grid intermediate said screen grid and that one of the signal grids which is farthest from said anode, and a pair of inductance coils connected in series to said anode-grid, each being magnetically couple-d respectively to one of the input signal circuits and to the common output circuit in such manner as to balance out the interaction ineach of said input circuits caused by the other circuit.

WILLIAM A. HARRIS. 

